Use this tool to find the **ERP** (Effective Radiated Power) from **EIRP** (Effective Isotropic Radiated Power).

## Formula

**ERP = EIRP/1.64 **

**ERP _{dBm} = EIRP_{dBm} – 2.15**

**Background**

**What is the difference between EIRP and ERP?**

**EIRP** is the power that would have to be radiated by an isotropic antenna to provide the same signal level as the actual source antenna in the direction of the antenna’s strongest beam.

**ERP** is the power that would have to be radiated by a half-wave dipole antenna to provide the same signal level as the actual source antenna in the direction of the antenna’s strongest beam.

In linear terms, the ratio of the gain of an isotropic antenna to that of a half-wave dipole antenna is given by the following relationship:

**G _{I}/G_{D} = 1.64**

Therefore on the Log scale **10*Log _{10}(G_{I}/G_{D})** =

**10*Log**

_{10}(1.64)or

**EIRP (dBm) – ERP (dBm) = 2.15** **dB**

**Applications**

FCC part 15 stipulates the EIRP limits in various frequency bands. Use the calculator on this page to convert the limits to those for ERP.

Wi-Fi equipment manufacturers will use RP-SMA connectors on Access Points. This is done to prevent customers from using high gain antennas (most of which have regular SMA connectors) to increase the EIRP which will in turn exceed the FCC limits.

**Related Calculators**

- EIRP is the sum of the transmit power in dBm minus losses due to connectors, cables and adapters plus the antenna gain in dBi.
- Half wave dipole antenna is a dipole with dimensions equal to half the operating wavelength. Convert wavelength to frequency and use this number to calculate the physical length of the dipole.